resolved: try to reduce number or DnsResourceKeys we keep around by merging them

Quite often we read the same RR key multiple times from the same message. Try to replace them by a single object when
we notice this. Do so again when we add things to the cache.

This should reduce memory consumption a tiny bit.

1 files changed, 40 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-rr.c b/src/resolve/resolved-dns-rr.c
index 02c6b239d5..7273ef3825 100644
--- a/src/resolve/resolved-dns-rr.c
+++ b/src/resolve/resolved-dns-rr.c
@@ -334,6 +334,46 @@ int dns_resource_key_to_string(const DnsResourceKey *key, char **ret) {
         return 0;
 }
 
+bool dns_resource_key_reduce(DnsResourceKey **a, DnsResourceKey **b) {
+        assert(a);
+        assert(b);
+
+        /* Try to replace one RR key by another if they are identical, thus saving a bit of memory. Note that we do
+         * this only for RR keys, not for RRs themselves, as they carry a lot of additional metadata (where they come
+         * from, validity data, and suchlike), and cannot be replaced so easily by other RRs that have the same
+         * superficial data. */
+
+        if (!*a)
+                return false;
+        if (!*b)
+                return false;
+
+        /* We refuse merging const keys */
+        if ((*a)->n_ref == (unsigned) -1)
+                return false;
+        if ((*b)->n_ref == (unsigned) -1)
+                return false;
+
+        /* Already the same? */
+        if (*a == *b)
+                return true;
+
+        /* Are they really identical? */
+        if (dns_resource_key_equal(*a, *b) <= 0)
+                return false;
+
+        /* Keep the one which already has more references. */
+        if ((*a)->n_ref > (*b)->n_ref) {
+                dns_resource_key_unref(*b);
+                *b = dns_resource_key_ref(*a);
+        } else {
+                dns_resource_key_unref(*a);
+                *a = dns_resource_key_ref(*b);
+        }
+
+        return true;
+}
+
 DnsResourceRecord* dns_resource_record_new(DnsResourceKey *key) {
         DnsResourceRecord *rr;